Dual connectivity refers to a wireless device that communicates using radio resources provided by at least two different network nodes (sometimes referred to as Master eNB (MeNB) and Secondary eNB (SeNB)) connected with non-ideal backhaul while the wireless device is in a connected state, such as a Third Generation Partnership Project (3GPP) long term evolution (LTE) user equipment (UE) in a RRC_CONNECTED state. A MeNB may also be referred to as an anchor node and the SeNB may also be referred to as a booster node. A wireless device in dual connectivity maintains simultaneous connections to the anchor and booster nodes. As the master node, the MeNB controls the connection and handover of SeNB. The SeNB does not perform handover on its own. To perform a SeNB change, the MeNB may signal a SeNB release and SeNB addition. Both the MeNB and SeNB can terminate the control plane connection towards the wireless device, thus both may be a controlling node of the wireless device.
The wireless device receives system information (e.g., SIBs) from the MeNB. In addition to the MeNB, the wireless device may be connected to one or more SeNB for added user plane support. In LTE, the MeNB and SeNB are connected via the Xn interface, which is currently the same as the X2 interface between two eNBs.
Specifically for LTE, dual connectivity is a UE operation mode in RRC_CONNECTED state where the UE is configured with a Master Cell Group (MCG) and a Secondary Cell Group (SCG). A MCG is a group of serving cells associated with the MeNB. The MCG may comprise a primary cell (PCell) and optionally one or more secondary cells (SCells). A SCG is a group of serving cells associated with the SeNB. The SCG may comprise a primary secondary cell (PSCell) and optionally one or more SCells. The MeNB terminates the S1-MME connection. The SeNB provides additional radio resources for the UE.
FIG. 1 is a block diagram illustrating an example dual connectivity network. Network nodes 220 serve one or more UEs 210. As one example of dual connectivity, UE 210a is operating in dual connectivity with network nodes 220a and 220b. Network node 220a is serving UE 210a as a MeNB, and network node 220b is serving UE 210a as a SeNB. As another example, UE 210d is operating in dual connectivity with network nodes 220c and 220d. Network node 220c is serving UE 210d as a MeNB, and network node 220d is serving UE 210d as a SeNB. In the illustrated examples, only one SeNB is connected to a UE. In general, however, more than one SeNB may serve a UE.
Dual connectivity is a wireless device specific feature, which means that a network node can support a dual connected wireless device and single carrier wireless devices at the same time. For example, in FIG. 1 network node 220a is a serving network node for UE 210b and a MeNB for UE 210a. Network node 220d is a serving network node for single carrier UE 210e and a SeNB for UE 210d. Typically, the dual links with MeNB and SeNB may belong to different carrier frequencies and even different frequency bands.
The role of master and secondary, or anchor and booster, are relative to a particular wireless device. For example, a network node may operate as an anchor for one wireless device and the same network node may operate as a booster for another wireless device. Thus, a network node may typically be capable of providing functions for both roles. For example, a wireless device operating in dual connectivity reads is system information from the anchor node. A booster node, however, may serve other wireless devices that are not operating in dual connectivity and may provide system information to those wireless devices.
In general, the MeNB provides system information, terminates the control plane, and may terminate the user plane. The SeNB terminates the user plane, and may terminate the control plane.
In one application, a wireless device is dual connected to two network nodes to increase its data rate by receiving data from both nodes. This may be referred to as user plane aggregation. User plane aggregation achieves similar benefits as carrier aggregation using network nodes that are not connected by a low-latency backhaul/network connection. Without a low-latency backhaul connection, the scheduling and HARQ-ACK feedback from the wireless device to each of the network nodes is performed separately. For example, a UE may have two uplink transmitters to transmit uplink control and data to the connected network nodes.
A dual connected wireless device may operate in a synchronized or unsynchronized mode. Because dual connectivity operation involves two non-co-located transmitters (i.e., MeNB and SeNB), one factor related to wireless device receiver performance is the maximum receive timing difference (Δt) of the signals from MeNB and SeNB received at the wireless device receiver. This timing difference is the reason for two modes of dual connectivity operation (synchronized and unsynchronized) with respect to the wireless device.
Synchronized operation refers to a wireless device that can perform dual connectivity operation provided the received time difference (Δt) between the signals received at the wireless device from the component carriers belonging to the MCG and SCG are within a certain threshold (e.g., ±33 μs). More specifically, synchronized operation means that the received time difference (Δt) at the wireless device between the subframe boundaries of the received component carriers belonging to the MCG and SCG are within a certain threshold (e.g., ±33 μs).
Unsynchronized operation refers to a wireless device that can perform dual connectivity operation regardless of the received time difference (Δt) between the signals received at the wireless device from the component carriers belonging to the MCG and SCG (i.e., for any value of Δt). More specifically, unsynchronized operation means that the received time difference (Δt) at the wireless device between the subframe boundaries of the received component carriers belonging to the MCG and SCG can be any value (e.g., more than ±33 μs, any value up to ±0.5 ms, etc.). A wireless device may be referred to as operating in synchronized or unsynchronized mode, or interchangeably as synchronous or asynchronous mode, respectively.
A wireless device may also be referred to as operating at a particular synchronization level. The synchronization level may be determined by the received time difference (Δt) between the signals received at the wireless device from the component carriers belonging to the MCG and SCG. For example, a wireless device with received time difference below a certain threshold, such as 33 μs, may be operating at a synchronous (or synchronized) level. A wireless device with received time difference above a certain threshold, such as 33 μs, may be operating at an asynchronous (or unsynchronized) level. The term synchronization type may refer generally to either a synchronization mode or level.
A wireless device may signal to the network node to indicate whether the wireless device is capable of synchronized and/or unsynchronized dual connectivity operation. The capability information may be associated with each band or band combination supported by the wireless device for dual connectivity operation. For example, a wireless device may indicate that it supports synchronized and unsynchronized dual connectivity operations for frequency band combinations, such as band 1+band 3, and band 7+band 8. Based on the received wireless device capability information, the network node can determine whether the wireless device should be configured for synchronized or unsynchronized dual connectivity operation for a particular band or band combination.
The maximum receive timing difference (Δt) at the wireless device consists of three main components: (1) relative propagation delay, which is expressed as the difference of propagation delay between MeNB and SeNB; (2) transmit timing difference related to synchronization levels between MeNB and SeNB antenna connectors; and (3) delay related to multipath propagation of radio signals from the MeNB and SeNB.
The relative propagation delay between MeNB and SeNB can be compared to the propagation delay in carrier aggregation. Carrier aggregation is designed for a maximum propagation delay of 30.26 μs for worst case non-co-located carrier aggregation coverage case. 30.26 μs corresponds to signal propagation distance of just over 9 km. In dense urban scenarios, the maximum misalignment related to propagation delay is typically around 10 μs. This is linearly related to the relative physical distance between the nodes. Thus, for dual connectivity, a large portion of the timing misalignment margin may not be required because of the shorter distance between nodes. This means the margin requirement may be relaxed to the synchronization accuracy between MeNB and SeNB (e.g., 3 μs). 3 μs is the co-channel synchronization accuracy requirement for TDD, which means that the tightest achievable requirement is 3 μs.
Another factor is the received time difference at the wireless device between signals from the MeNB and SeNB. For synchronized operation, the MeNB and SeNB transmit timing is synchronized to a particular level of time accuracy. For asynchronous operation, the synchronization accuracy may be any variance up to 1 ms, which is more relaxed than synchronized operation. The receive timing difference is the received timing misalignment between two received signals from subframe boundaries of MeNB and SeNB at the wireless device. It does not refer to the transmit timing mismatch levels between the MeNB and SeNB.
FIG. 2 is a block diagram illustrating maximum receive timing difference at a wireless device. The top example illustrates subframes transmitted from a MeNB and SeNB in synchronized operation. The bottom example illustrates subframes transmitted from a MeNB and SeNB in unsynchronized operation.
The maximum receive timing difference (MRTD) is illustrated by the arrow marking the time difference between the start of the subframe received from the MeNB and the start of the subframe received from the SeNB. In the top example (synchronized operation), the MRTD is within the range of 33 μs. In the bottom example (unsynchronized operation), the MRTD can be any value less than 1 ms.
For dual connectivity using dual transmit/receive components and a non-ideal backhaul, the MeNB and SeNB may typically not be synchronized with each other. Dual transmit/receive components mean that the separate links may be served by separate power amplifiers, which makes synchronization unnecessary. A wireless device that is capable of unsynchronized operation is likely also capable of synchronized operation.
Another factor is related to multipath delay in the radio environment. The received time difference of radio signals from MeNB and SeNB may include delay introduced by the multipaths on individual links based on the radio environment characteristics. For example, in a typical urban environment the delay spread of multiple paths received at the wireless device may typically be on the order of 1-3 μs. In wide areas like in suburban or rural deployment scenarios, however, the channel delay spread caused by multipath effect of the signals observed at the wireless device is relatively smaller (e.g. less than 1 μs).
In general, network-wide synchronization is not needed for dual connectivity because dual connectivity is a wireless device specific operation. A particular wireless device is connected to two network nodes in dual connectivity operation, thus any synchronization requirement between the two network nodes is only relevant when they serve the particular wireless device for dual connectivity operation (i.e., when the two network nodes are performing as MeNB and SeNB for the particular wireless device).
As described with respect to FIG. 1, the same MeNB and SeNB may also be serving other wireless devices not operating in dual connectivity. Thus, strict synchronization requirements between MeNB and SeNB are not necessary. However, for a wireless device operating in dual connectivity to receive signals from MeNB and SeNB within the maximum allowed received time difference, the wireless device needs to support particular requirements (e.g., measurement requirements, measurement accuracy requirements, RLM requirements, UE performance requirements, UE demodulation and CSI requirements, etc.). For example, the received time difference at the wireless device from the MeNB and the SeNB should be within a particular time limit, and the maximum transmit time difference between the MeNB and the SeNB should be within a particular time limit.
A wireless device may use timing measurements to estimate the received time difference between the MCG and the SCG. The wireless device may determine received time difference between the MCG and SCG based on a measurement which may be referred to as system frame number (SFN) and subframe time difference (SSTD) or SFN and subframe timing offset. The wireless device may use the SSTD autonomously and/or report it to one or more network nodes. The SSTD is defined by the following equation:SSTD=[(SFNi−SFNj)*327200+x]*Ts Where SFN indices in MeNB and SeNB are denoted with i and j, respectively; Ts is a basic time unit (Ts≈32.5 μs); and SFN and subframe time offset (Δt) is expressed as illustrated in FIG. 3.
FIG. 3 is a block diagram illustrating system frame number and subframe time offset estimation in SSTD measurement. Consecutive frames of the MeNB are labelled with system frame numbers i−1, i, i+1, and i+2. Consecutive frames of the SeNB are labelled with system frame numbers j−1, j, j+1, and j+2. The offset between the start of MeNB frame i and SeNB frame j is denoted as Δt.
LTE also specifies a timing relationship between uplink and downlink transmissions. Timing advance (TA) is a negative offset, at the wireless device, between the start of a received downlink subframe and a transmitted uplink subframe. This offset at the wireless device enables the downlink and uplink subframes to be synchronised at the network node.
FIG. 4 is a block diagram illustrating uplink-downlink timing relations. FIG. 4 illustrates a downlink radio frame and a corresponding uplink radio frame for system frame number i. The start of uplink radio frame i precedes the start of downlink radio frame i.
More specifically, The transmission of the uplink radio frame number i from the wireless device starts (NTA+NTA offset)×Ts seconds before the start of the corresponding downlink radio frame at the wireless device, where:
0≤NTA≤20512 and NTA Offset=0 for frame structure type 1 (i.e., LTE FDD); and
0≤NTA≤20512 and NTA offset=624 for frame structure type 2 (i.e., LTE TDD).
The value of 624 Ts corresponds to 20 μs.
The uplink timing advance is maintained by the network node through timing advance (TA) commands sent to the wireless device. The network node may determine the timing advance, for example, based on measurements on uplink transmissions from the wireless device. Timing advance updates are signalled by the network node to the wireless device in MAC PDUs. For a TA command received in subframe n, the wireless device makes the corresponding uplink transmission timing adjustment at the beginning of subframe n+6.
Two network nodes located at different locations (non-collocated) may experience different propagation delays with respect to a wireless device, and thus the wireless device may use a different timing advance for uplink transmission to the two network nodes. Because maintaining a separate timing advance for each serving cell can be impractical, a timing advance group (TAG) consists of one or more serving cells with the same uplink timing advance and the same downlink reference cell.
The timing advance command for a TAG indicates the change of the uplink timing relative to the current uplink timing for the TAG as multiples of 16 Ts, where Ts=32.5 μs and is referred to as a basic time unit in LTE. A wireless device configured with at least two uplink serving cells (e.g., PCell and SCell) may be configured with two TAGs: pTAG and sTAG for a wireless device configured for uplink carrier aggregation, and pTAG and psTAG for a wireless device configured for dual connectivity operation. The same timing advance command is applicable for all serving cells in the same TAG (e.g., TA1 for PCell and SCell(s) belonging to pTAG, and TA2 for all SCell(s) belonging to sTAG).
If a wireless device is configured with inter-band carrier aggregation and also with multiple TAGs (e.g., pTAG and sTAG), then the wireless device is required to handle a maximum uplink transmission timing difference (ΔT) between the pTAG and the sTAG of at least 32.47 μs. A wireless device configured with pTAG and sTAG may stop transmitting on the SCell if, after a timing adjustment based on a received timing advance command, the uplink transmission timing difference between PCell and SCell exceeds the maximum value the wireless device can handle (i.e., 32.47 μs).
For dual connectivity, the value of ΔTAG depends on whether the wireless device is operating with synchronous or asynchronous dual connectivity. The values of maximum allowed ΔTAG can be up to 35.47 μs and 500 μs, depending on whether the wireless device is operating with synchronous or asynchronous dual connectivity. The MCG and SCG may likely use different duplex modes (e.g., MCG uses FDD and SCG uses TDD, or vice versa), in which case the maximum allowed ΔTAG may be larger. This is because of the 624 Ts (=20 μs) time offset in TDD between uplink and downlink frame timing, as illustrated in FIG. 4 described above. For example, the ΔTAG for synchronous TDD-FDD dual connectivity may be up to 55.47 μs.
A particular problem with dual connectivity, however, is that the wireless device receives the uplink control information (UCI) approximately 4 ms before the wireless device is to perform the actual uplink transmission. For a wireless device using ePDCCH, the wireless device waits until the end of subframe Q−4 for decoding the UCI information, which provides allocations for actual uplink transmission in uplink subframe Q. After applying the timing advance at the wireless device, the actual wireless device uplink transmission occurs after (3-TA) ms in any single carrier transmission. LTE specifications require a UE to support a maximum of 0.67 ms of maximum timing advance. Thus, the minimum available time for uplink processing (e.g., encoding a transport block) is reduced to 2.33 ms. Dual connectivity has a maximum of 0.5 ms subframe timing boundary mismatch, thus the minimum available processing time at the wireless device may be significantly smaller than 2.33 ms. This may impact a wireless device implementation, because performing the same transmit operations under the reduced time constraint requires significantly higher processing power.